When a coral is broken or wounded, it releases highly reactive atoms of oxygen known as free radicals to close up the gashes.

But these powerful molecules can also inadvertently kill off some of the coral's healthy cells. Hydrogen peroxide, for instance, is a common free radical in corals, and it can damage every part of the cell, from DNA to proteins.

Hurt corals have also been known to take on brightly colored glows, noted study leader and coral immunologist Caroline Palmer. Wounds on Acropora millepora corals appear blue, for example, while injured tissues on Porites species—like the raised and swollen patches seen above—are an "intense" bubble-gum pink.

To figure out why damaged corals glow, Palmer and colleagues took small fragments from seven species of healthy wild Caribbean corals as well as samples of injured and healthy tissue from coral colonies on Australia's Great Barrier Reef.

The researchers then studied the intensity of the glow coming from healthy and wounded corals. What they found is that corals with a brighter glow are best able to keep free radicals from damaging healthy cells.

The glow is a result of so-called fluorescent proteins, which in corals act as antioxidants to keep free radicals at bay, Palmer's team says.

As invertebrates—a group of less complex creatures, including worms and snails—corals were thought to have very simple immune systems, said Palmer, whose study appeared October 6 in the journal PLoS One.

"However," she said in an email, "it's becoming evident that they have a wide range of defensive responses."